Cross-talk square-wave?

[Paul Koning]

> On Mar 29, 2017, at 1:17 PM, Chuck Guzis via cctalk <cctalk at classiccmp.org> wrote:
> 
> On 03/29/2017 07:08 AM, Noel Chiappa via cctalk wrote:
>>> From: Dwight Kelvey
>> 
>>> Is there any load resistance at the end of the line?
>> 
>> Yes, 270K to ground (i.e. pretty large). How does that have an effect
>> on whether cross-talk can create a square wave? Sorry, I'm not
>> understanding.
> 
> 1v across 270K represents 3.7 microamps, which isn't much, particularly
> at 25MHz. (I assume that you're using SPI to access the card, but the
> observation still holds).  And if you're using SPI, have you installed
> pullups on unused pins?
> 
> SD cards can be very noisy devices--remember that they have "smarts"
> inside, so they're not passive devices.
> 
> I'd go to interleaved ground cable or UTP for the device.     Also, make
> sure that your 3.3V supply is adequately decoupled--an SD card can draw
> somewhere n the neighborhood of 100ma when operating, if the datasheets
> are to be believed.   I use a separate 3.3V LDO regulator at the SD card
> socket.

SD cards are not SPI, they are a variation of MMC.  As with other interconnects, it's good to consider the connections as transmission lines, and look at what termination is expected.  270k seems like a rather strange value, it certainly can't be a termination and it isn't a plausible pulldown either.  The SD spec should explain what is expected; I knew it at one time but forgot by now.

	paul